Cement slurries are used to support the intermediate casing pipe in geothermal wells, and to protect the casing from corrosive fluids and gases. The environment in which these cement slurries are used are subject to high hydrostatic pressures (up to 2,000 psi) and to high temperatures (up to about 350.degree. C.).
Many methods and agents have been used to increase or improve the mechanical, physical, and microstructural characteristics of the cementitious compositions. One such method involves the use of reinforcing substances such as fibrous materials. Inorganic, mineral and organic-type fibrous materials, as represented by carbon, glass, and polyaramid fibers, have previously been evaluated for application as reinforcing materials in high-temperature lightweight cement matrix composites. Generally, the use of these high-performance and high-modulus fibers yields durable products capable of withstanding high air temperature operating conditions. However, when glass and polyaramid fibers embedded in cement matrices are exposed to a temperature of 300.degree. C. or higher, the fibers are very susceptible to chemical decomposition by the strong alkaline media of cement slurries. Scanning electron microscopy (SEM) examination of glass fiber surfaces treated with Ca(OH).sub.2 -saturated solutions at 300.degree. C. reveals a morphological change on the surface; the fiber surfaces are surrounded by a reticular network structure of calcium silicate hydrate that was precipitated in the vicinity of the surface. The glass fiber, therefore, becomes fragile, leading to physical disintegration of the fiber-reinforced geothermal cement composites. Likewise, polyaramid fibers exposed to an alkaline solution at 300.degree. C. also quickly deteriorate into small segments as a result of chemical disintegration of the organic macromolecule, thereby reducing the mechanical strength of the composite specimens.
Conversely, the addition of an adequate amount of carbon fiber to the autoclaved lightweight cement results in an increase in the mechanical properties. However SEM examination of the fracture surfaces reveals a fiber debonding-failure mechanism which appears to be due to poor bonding between the fiber and cement matrix. The observed low reinforcement efficiency of the fiber appears to be directly related to low interfacial shear strength. Therefore, good adhesion of the cement to the carbon fiber surface is needed in order to obtain efficient stress transfer. The ideal interfacial bond should be strong enough to hold the fibers and to allow a crack to propagate through the matrix without significant fiber pull out. On the other hand, an extraordinarily developed bond strength will produce a more brittle composite as a consequence of a decrease in the frictional stress transfer between the fiber and the matrix.
It is therefore an object of the present invention to provide a fiber-reinforced cementitious composition suitable for use in geothermal wells.
It is also an object of this invention to provide a fiber-reinforced cementitious composition with improved interfacial bonding between the fiber and the cement matrix.
It is also an object of this invention to modify the surface of the reinforcing fiber in order to promote adhesion of the fiber to the cement matrix.
It is also an object of this invention to provide reinforcing fibers which have been subject to oxidative treatment on the fiber surfaces. The oxidation of the fiber surface progressively introduces chemically active oxygen groups on the fiber surfaces. These oxygen groups preferentially react with neutralizing aluminum and calcium ions dissociated from the cement, thus forming an interfacial ionic reaction between the fiber surface and the cement matrix.
These objects and others will become evident from the following description .